Molded, plastic pressure tanks, such as are used in the water softener industry, have been made by laying up glass fiber matting in a confining mold, impregnating the matting with a suitable synthetic resin, and curing the resin while pressurizing the matting inside the mold with an inflatable bag. Another known procedure has involved laying up windings of reinforcing strands on a collapsible mandrel or inflatable bag, impregnating the windings with resin, and curing the resin to form a reinforced, cylindrical tank wall. Both molding techniques are expensive and often difficult to carry out successfully. The molding problems include shifting of the matting or windings in the mold and entrapped air in the resin, etc. all of which can adversely affect burst strength and tank life. Another disadvantage is the general inability to conveniently mold ports, threads, internal bosses, ribbing, and other detail.
For some applications it is desirable to mount structure such as distribution conduits within the tank. When the entire tank is molded as an integral unit, access to the interior of the tank is severely restricted. Access is normally provided by an inlet/outlet opening that is typically molded at one end of the tank. Any components mounted within the tank must therefore be smaller than the inlet/outlet opening.
Attempts have been made in the past to produce pressure tanks by injection molding them in sections and then bonding the sections together in a separate operation. The tanks produced in this manner have been characterized by a butt joint formed between overlapped edge portions of the molded sections. The typical butt joint is formed by an annular recess in the edge of the inner one of the overlapping sections. The recess is closed by the overlapping outer section to form an annular pocket that captures or encloses a fusion bonding filler material.
The geometry of the typical butt joint severely limits the strength of the finished tank. Bursting pressure tests have shown that failure at the joint is circumferential rather than longitudinal. This indicates that failure is not due to hoop stress, but rather to a combination of axial and bending stresses. The failure inducing bending stress is believed to be the result of a radial misalignment between the axial wall force and the reaction force through the joint.